Hydraulic fracturing has been utilized for many years to stimulate the production of oil, gas or other formation fluids from subterranean formations. In hydraulic fracturing, a suitable fluid is introduced into the down-hole formation by way of a well bore under conditions of flow rate and pressure which are at least sufficient to create or extend a fracture into a desire portion of the formation. Various fluids have been utilized in hydraulic fracturing. Most fluids utilized today, however, are aqueous-based liquids.
The presence of clay in oil and gas producing formations poses a problem for production from wells completed in such formations. Ordinarily, such clays are inert in the formation and do not disrupt the flow of hydrocarbons. When disturbed, however, by aqueous-based fluids used in well stimulation for example, clay particles can swell and reduce formation permeability.
Clay swelling problems in the past have been addressed by preflushing with slugs of salt-containing water and using inorganic salts in the aqueous stimulation fluid. Quite often the salt of choice has heretofore been potassium chloride (KCl) which converts the clay to a less swellable form by cation exchange with Na.sup.30 ions present on the clay surfaces. Other salts include calcium chloride, ammonium chloride and the like, typically dissolved in an aqueous preflush and/or in the aqueous stimulation fluid used for the formation treatment.
Clays dispersed throughout oil-producing formations may be described as stacked platelets with a net positive charge associated with the four short dimensional sides and a net negative charge associated with the two long dimensional faces. It is generally believed that the concept of surface charge may be used to understand the mechanisms involved in swelling inhibition. When the large negatively charged face or surface is exposed to an aqueous solution, it attracts cations from the solution. In order to inhibit the swelling phenomenon, minimization of the hydratable surface area of the clay is necessary. One way that this may be accomplished is by flocculating and decreasing the surface charge density, or by increasing the ionic strength of the aqueous phase, or both. By allowing cations with small charge-to-surface-area ratios to associate with the particle, the effective strength of the negatively charged, double-face platelet layer surfaces will be diminished, allowing greater platelet-platelet interaction. Increasing the ionic strength of the solutions will also have the same effect.
In the case of potassium chloride, it is generally believed that the potential for clay swelling is shunted via a cation exchange of potassium ions for the more hydration-enticing native cations, for instance sodium. It has been found that K.sup.+ is much better at creating electrostatic links between the negatively charged faces of the stacked clay platelets than the abundant Na.sup.+, therefore allowing less osmotic migration of water to occur between the platelets. While a lower concentration of K.sup.+ ions relative to Na.sup.+ ion is needed to flocculate clays, NH.sub.4.sup.+ ions have been shown to be even better or equal to K.sup.+ ions in creating electrostatic links and reducing osmotic migration of water.
While salts may be effective in protecting the formation, several problems are associated therewith: (1) the amount of material needed for preparing an effective fluid may be very high and it is often difficult to dissolve such solid components in the treating fluids in the quantities required; (2) in environmentally sensitive areas, there may be limits on the permissible amount of chloride; (3) the presence of salts may interact with other additive components of the aqueous stimulation fluid, such as, for example, viscosifying agents, the hydration of which is inhibited by such salts; and (4) the duration of the stabilizing effect thereof generally cannot be tailored to meet the optimum duration for a given situation. Accordingly, there is a need for a down-hole clay stabilizing composition that is more inert to other down-hole fluid additives, lower in chloride ion and therefore more environmentally tolerable, which has greatly enhanced clay stabilizing effectiveness compared to potassium chloride and other similar salts, and which may be tailored as to the duration of stabilizing effect. The objects of the present invention are to provide a clay stabilizing composition having such advantageous features and other advantages as described below.